In our solar system, only one world has just the right size, just the right temperature and just the right home for liquid water. Long ago, at least one other rocky planet had water, too, but it’s gone now; Mars dried up and turned to rust. A distant moon called Europa has a lot of water, but too far from the sun’s warmth, the moon remains frozen. Closer in, Mercury and Venus are too hot to keep it.

"Between the fire and the ice is this green zone, the habitable zone, where a planet could have liquid water on its surface," said William Borucki, principal investigator on the planet-hunting Kepler Space Telescope. In that zone, conditions are right for thick greenhouse atmospheres, and for planets with sufficient gravity to hold on to them.